Compared with the liquid crystal display panel in the conventional technology, the OLED (Organic Light Emitting Diode) display panel has characteristics such as faster response speed, better color purity and brightness, higher contrast, wider visual angle and so on. Thus, the display technology developers are paying increasingly widespread attention to the OLED display panel.
A pixel unit in the OLED display panel mainly includes an organic light emitting diode and a pixel unit driving circuit for driving the organic light emitting diode. The traditional 2T1C pixel unit driving circuit is shown in FIG. 1A or FIG. 1B: including a first transistor T1, a driving transistor DTFT and a storage capacitor Cst. The first transistor T1 is controlled by a first scan signal Sn output from a scan line, so as to control the writing of a data signal Data of a data line. The driving transistor DTFT is used for controlling the organic light emitting diode OLED to emit light. The storage capacitor Cst is used to provide a holding voltage to the gate electrode of the driving transistor DTFT.
The organic light emitting diode OLED can emit light while it is driven by the driving current produced when the driving transistor DTFT is operating in saturation state, wherein the driving current IOLED can be expressed as:
      I    OLED    =            1      2        ⁢                  μ        n            ·              C        OX            ·              W        L            ·                        (                      Vdd            -            Vdata            -            Vth                    )                2            
where, μn·Cox·W/L is a constant related to process and driving design, for example, μn is the carrier mobility, Cox is the gate oxide layer capacitance, W/L is width to length ratio of the transistor; Vdata is the voltage of the data signal Data, Vdd is the driving voltage of the driving transistor DTFT and is shared by all the pixel units, Vth is the threshold voltage of the driving transistor DTFT.
However, due to technical limitations, the uniformity of the threshold voltage Vth is usually poor, and drifting of the threshold voltage Vth will occur in use. From the above equation it can be known that, if different pixel units have different Vth, then there is a difference in the driving current, causing non-uniform of the display brightness; if the threshold voltage Vth of the driving transistor drifts over time, it may result in different currents over time, influencing the display effect. Further, since there is a certain impedance on the driving voltage line inevitably, so there is a certain voltage drop (i.e., IR Drop) when the driving voltage Vdd of the driving transistor transmits from one end of the OLED display panel to the other end, which will result in the display brightness non-uniform across the OLED display panel, influencing the user's experience.